1. Field of the Invention
This invention relates generally to a system for enhancing signal coverage inside building structures and, more particularly, to an antenna system for controlling the duration of local signal nulls in a microcellular communication system to permit signal data recovery through conventional error-correction techniques.
2. Discussion of the Related Art
The use of radio frequency signals for indoor data or voice communications is increasingly desirable because of improvements in channel capacity and related apparatus cost reductions. Within an office building, a warehouse, a factory, a hospital, a convention center or an apartment building, radio communication avoids tying the user to particular locations within these buildings, thus offering true mobility, which is convenient and perhaps necessary. Availability of radio links indoors also drastically reduces wiring requirements for new construction and offers flexibility for changing or creating various communication services within existing buildings without the conventional expense of rewiring the structure. The problems associated with indoor radio communication systems include how to offer the sophisticated local radio communication system necessary to provide such services to the majority of people within a building simultaneously. Such systems involve radio signals that are strongly affected by the multipath delay spread and the spatial and temporal statistics of signal attenuation particular to indoor propagation environment.
The indoor signal propagation environment has been examined by several practitioners in recent times. For instance, Jean Francois LaFortune et al. ("Measurement and Modeling of Propagation Losses in a Building at 900 MHz", IEEE Trans. Veh. Technol., Vol. 39, No. 2, pp. 101-108, May 1990) offer an empirical attenuation model giving estimates of transmission, reflection and diffraction phenomena occurring in the transmission path based on measurements in two large buildings of similar design. Also, Adel A.M. Saleh, et al. (A Statistical Model For Indoor MultiPath Propagation", IEEE Jour. Sel. Areas Commun., Vol. SAC-5, No. 2, pp. 128-137, Feb. 1987) report that indoor multipath propagation measurements support a simple statistical multipath model of the indoor radio channel. With this model, the received signal rays arrive in clusters. The rays have independent uniform phases and independent Rayleigh amplitudes with variances that decay exponentially with cluster and ray delays. These clusters and the rays within each cluster form Poisson arrival processes with different but fixed rates. The clusters are formed by the building superstructure, while the individual rays are formed by objects in the vicinities of the transceivers.
A fundamental problem with the indoor radio channel arises from the signal fading characteristics resulting from the multipath propagation statistics. Practitioners have introduced digital communication systems designed to correct dropout errors resulting from transient signal fading. For instance, direct-sequence spread-spectrum microcellular systems have been proposed for indoor radio communications (M. Kavehrad, et al., "Design and Experimental Results For A Direct Sequence Spread Spectrum Radio Using DPSK Modulation For Indoor, Wireless Communications" IEEE Jour. Sel. Areas Commun., Vol. SAC-5, No. 5, pp. 815-823, Jun. 1987). Code Division Multiple Access (CDMA) techniques are also proposed to provide several simultaneous bi-directional links to a plurality of mobile stations from a single base station within a building (G. L. Turin, "The Effects of Multipath and Fading On The Performance of Direct-Sequence CDMA Systems," IEEE Jour. Sel. Areas Commun., Vol. SAC-2, No. 4, pp. 597-603, Jul. 1984). One solution to combat indoor multipath fading in CDMA systems is to increase the spreading bandwidth in combination with a the RAKE receiver, which reduces multipath fading effects (J. S. Lehnert, "Multipath Diversity Reception of Spread-Spectrum Multiple-Access Communications," IEEE Trans. Commun., Vol. COM-35, No. 11, pp. 1189-1198, Nov. 1987).
A RAKE-type receiver architecture provides multiple receivers each capable of receiving a signal that has traveled a different path and therefore exhibits a different delay. An example of such a RAKE receiver is disclosed in U.S. Pat. No. 5,109,390 entitled "Diversity Receiver In A CDMA Cellular Telephone System" assigned to the assignee of the present invention, the disclosure of which is incorporated by this reference. Included in the described receiver is a separate searcher receiver which continuously scans the time domain looking for the best paths and assigning the multiple receivers accordingly. The receivers can track distinct arriving signals provided that the time difference between arriving signals exceeds one PN chip duration, i.e. 1/bandwidth of the spread spectrum signal. In an outdoor cellular system, delays greater than one PN chip are likely due to the relatively large distance between reflective objects. However in an indoor system, multipath signals are likely to be reflected from closely located object and therefore have short time delays with respect to each other.
The signal fading characteristics of indoor radio channels result from multipath propagation due to reflections from closely located surfaces. It is known in the art that motion of people causes transient fading at rates less than 5 Hz in signal carrier frequencies of around 900 MHz. On a lesser scale, it is also known in the art that observations of continuous fading at 120 Hz are related to the effects of the electric power network manifested in fluorescent light plasma columns (P. Melancon et al. "Effects of Fluorescent Lights on Signal Fading Characteristics For Indoor Radio Channels", Electron. Lett., Vol. 28, No. 18, pp. 1740-1741, Aug. 27, 1992). The fading caused by fluorescent lighting varies continuously at a significantly higher rate than fading caused by the normal movements of people. The average signal-to-noise ratio (SNR) is reduced and the fading signal is manifested at a rectified sine wave that is always present at any location where there are fluorescent lights.
One solution to the multipath fading problem was proposed by Gilhousen et al. in U.S. patent application No. 07/624,118 filed on Dec. 7, 1990, entitled "CDMA Microcellular Telephone System and Distributed Antenna System Therefor", assigned to the assignee hereof and entirely incorporated herein by this reference. This method uses an array of cell-site transceiver antennas located at different sites or "sectors" within the cell area. The single base station signal is both transmitted and received from all of the antennas in the array. The signal at each antenna is substantially delayed in time with respect to the signals at the other antennas so that each antenna's signal can be discriminated through its temporal diversity by a receiver. This solution works well for multipath fading because the signal is unlikely to fade at all cell antennas simultaneously. Thus, the multiple-site signals can be combined through temporal diversity to create a nonfading aggregate signal. Although the just mentioned distributed antenna system provides significant improvement in the indoor cellular environment, other factors exit which can cause a degradation in system performance.
In an indoor CDMA cellular system, a cell-site or base station transceiver establishes independent communication with a plurality of mobile transceivers. The transmit and receive frequencies for the base station transceiver are different. Although different, the two frequencies are within the same band and the path loss of the base station transceiver to mobile transceiver link or forward link is an excellent predictor of the path loss of the mobile transceiver to base station transceiver link or the reverse link. Therefore, typically, the mobile transceiver measures the signal level received from the base station transceiver and bases the level of its transmitter signal thereon. This operation is referred to as open loop power control for which further details can be found in U.S. Pat. No. 5,056,106 entitled "Method and Apparatus for Controlling Transmission Power in a CDMA Cellular Mobile Telephone System" issued Oct. 8, 1991, the disclosure of which is incorporated by this reference. In the CDMA system, power control is critical to achieve theoretical maximum capacity in the system. The transmit power level of all mobile transceivers must be closely controlled such that the transmitted signals arrive at the base station transceiver at the same level. Typically the mobile stations transmit at a minimum power level sufficient to maintain a quality communication link. To supplement open loop power control, a closed loop power control is disclosed in above mentioned U.S. Pat. No. 5,056,106. In closed loop power control, the base station sends transmission power adjustment commands to the of a mobile station on the forward link thereby controlling the mobile stations transmitted power on the reverse link.
In an indoor cellular environment, random and severe multipath fading occurs. Open loop power control is directed to compensate for such fading condition. In the outdoor environment, there is a good correlation between fading on the transmit and receive frequency band. However in the indoor cellular environment fading can be quite different for the transmit frequency than for the receive frequency. Uncorrelated fading of these two signals can cause improper power control adjustments in the open loop power control. These improper adjustments can result in unwanted fluctuations in the signal level received at the base station transceiver from the mobile transceiver thereby affecting the capacity of the system. For instance, the difference in the forward link and the reverse link fading may cause the mobile station to exceed the possible range of closed loop power control. Therefore it is desirable to have an antenna system which reduces fading problems and the deleterious effects of fading on power control.